Process analytic sensors and instruments are employed in a variety of industries to measure and control gasses and liquids. Some process analytic sensors, such as oxygen sensors, utilize a heated solid electrolyte cell to measure a gas species of interest. As used herein, the term solid electrolyte cell means a quantity of the solid electrolyte, for example zirconia-yttria ((ZrO2)(1-x)(Y2O3)x), and porous electrical contacts or electrodes connected thereto, usually made of platinum or other suitable materials.
Gas analyzers having a probe with a solid electrolyte cell to measure gaseous oxygen are well known. In one commercially available sensor (See U.S. Pat. No. 3,928,161) two porous platinum electrodes are deposited on opposite sides of the porous zirconia solid electrolyte. A common application for such analyzers is the measurement of gaseous oxygen in a flue or duct such as a smokestack. The response of the sensor to the differential oxygen concentrations with the reference electrode partial pressure fixed, e. g. air, can be calculated on the process side using the Nernst equation:
                              EMF          =                                                                      (                                      RT                                          4                      ⁢                                                                                          ⁢                      F                                                        )                                ⁢                                  ln                  ⁡                                      (                                                                  P                        process                                                                    P                        ref                                                              )                                                              +              C                        =                                                            S                  ⁢                                                                          ⁢                                      log                    ⁡                                          (                                                                        P                          process                                                                          P                          ref                                                                    )                                                                      +                C                            =                                                0.496                  *                  T                  *                                      log                    ⁡                                          (                                                                        P                          process                                                                          P                          ref                                                                    )                                                                      +                C                                                    ,                            Eq        .                                  ⁢        1            
C is a cell constant, S is a cell slope which is a function of cell temperature T, and Pprocess and Pref are the oxygen partial pressure at a measurement and reference end, respectively, of the solid electrolyte cell. Actual solid electrolyte cells deviate from Equation 1 to some extent after aging in the combustion process.